This research covers several areas of study of the structure of the murine Ia antigens, which are integral membrane glycoproteins encoded by genes in the I-region of the H-2 complex (MHC) of the mouse. The first objective is the elucidation of the structures of the oligosaccharides carried by Ia antigens. The structures of the carbohydrate moieties of the alpha, beta and Ii chains of the I-Ak antigen will be determined, in as much detail as possible, using radiochemical tech-niques. The second objective is the purification of milligram amounts of Ia antigens using affinity chromatography. The third objective is the localization of the antigenic determinants of the I-Ak and I-Ek antigens. Monoclonal antibodies will be used to define the relationship betweenthe antigenic structure and the primary structure of the Ia antigens of the k haplotype since a large number of reagents are available for these antigens. The fourth objective is to carry out a detailed examination of the orientation of the Ia antigens in the lipid bilayer of the plasma membrane, including the development of markers for the extracellular, cytoplasmic and membrane-integrating portions of the molecules. Within this general objective are experiments designed to search for potential interactions of Ia antigens with other membrane proteins, or with cytoskeletal or cytoplasmic elements. All of the proposed studies fall within the general disciplines of immunology and biochemistry. They derive their significance from two different considerations: (1) It is increasingly apparent that the Ia antigens are the Ir gene products and, as such, they occupy a central role in the regulation of the immune response. (2) There are a growing number of disease susceptibilities in humans that are linked to specific alleles of HLA loci, including HLA-D, the human equivalent of the I-region. Although the mechanisms underlying these associations are not known, it is clear that developing a full understanding of the biochemistry of the MHC gene products, coupled with developing a detailed model for their role in immunoregulation, is the most promising method for determining the basis for these associations in humans.